FEMTOSECOND RELAXATION OF CARRIERS GENERATED BY NEAR-BAND-GAP OPTICAL-EXCITATION IN COMPOUND SEMICONDUCTORS

A detailed examination of the physics underlying femtosecond relaxatio n of optically excited carriers in the near-band-gap regime of compoun d semiconductors with a special emphasis on band renormalization and C oulomb enhancement of the optical matrix elements is presented. This i s done using a Monte Carlo formulation including Coulomb enhancement, band renormalization, and dynamic screening. The accuracy of the simul ation has been verified through correlation with a series of experimen ts performed over a wide range of near-band-gap photon energies and pu lse intensities. The results are found to differ greatly from those ob tained for energy excitations far from the band edge. The observed car rier relaxation is found to be very insensitive to all relative scatte ring rates in contrast to excitation at high energies. Coulomb enhance ment and band renormalization together are found to be important facto rs at both low and high excitation energies and should be important co nsiderations in all efforts in the field. For a cold distribution, the effect of these processes is to accelerate the observed relaxation wh ile for a hot distribution the opposite is found. These two processes are unimportant if the carriers are excited near thermal energies. The insensitivity of the simulation to relative contributions of the scat tering processes, in combination with the strong distortions introduce d by band renormalization and Coulomb enhancement makes the extraction of scattering-rate information difficult. Thus, near-band-gap relaxat ion should rather be considered a probe into band renormalization, Cou lomb enhancement, and screening. The good correlation between measured and simulated data provides justification for extending the present s emiclassical formulation to the computation of macroscopic physical ob servables dependent on near-band-gap femtosecond carrier relaxation.